Abstract

The freshwater benthic pearl clam, Hyriopsis schlegeli, was experimentally exposed to Cryptosporidium parvum oocysts, and it was verified that the oocysts were eliminated predominantly via the fecal route, retaining their ability to infect cultured cells (HCT-8). The total fecal oocyst elimination rate was more than 90% within 5 days after exposure to the oocysts. H. schlegeli was able to survive in the final settling pond of a sewage plant for long periods, as confirmed by its pearl production. In the light of these findings, the clam was placed in the final settling pond in a trial to test its long-term efficacy in depleting oocysts contaminating the pond water. The number of clams placed was set to ensure a theoretical oocyst removal rate of around 50%, and the turbidity and the density of feed microbes in the overflow trough water of the pond were about 35% and 40 to 60% lower, respectively, than in the control water throughout the year. It was found that the clam feces containing oocysts were sufficiently heavy for them to settle to the bottom of the pond, despite the upward water flow. From these results, we concluded that efficient depletion of oocysts in the sewage water of small or midscale sewage treatment plants can be achieved by appropriate placement of H. schlegeli clams.

Relative amounts of C. parvum oocysts detected in the excreta and GI tracts of H. schlegeli clams after a single dose of oocysts at 6.67 × 104/clam. The data are means ± standard deviations for five replicates. The absence of bars indicates that the error was smaller than the symbol. (A) Percentage of C. parvum oocysts detected in feces. (B) Percentage of C. parvum oocysts in the GI tract. (C) Integrated recovery of oocysts in feces.

Photographs of developmental stages of C. parvum prepared from fecal samples of H. schlegeli clams in HCT-8 cells 2 days after inoculation. (A) Fluorescence micrograph of a field of meronts 2 days after inoculation with sporozoites prepared from a fecal sample. (B) Normarski interference-contrast photomicrograph of the corresponding field in panel A.

Reductions of C. parvum oocyst concentrations and water turbidity caused by the clams and rates of water filtration by the clams at various water temperatures and feed densities. Means and standard deviations are shown based on results from seven replicates. The absence of bars indicates that the error was smaller than the symbol. (A) Results obtained using laboratory water at 20°C. ●, oocyst density with both clams and feed (28 mg/liter); ○, oocyst density with clams but without feed; , oocyst density without clams or feed; ▲, turbidity with both clams and feed; , turbidity without clams but with feed. (B) Results obtained using final settling pond water at 20°C. ●, oocyst density with clams; , oocyst density without clams; ▲, turbidity with clams; , turbidity without clams. (C) Water filtration by clams at various water temperatures and feed densities. See Materials and Methods for details of calculation of the water filtration rate (in L/h).

Time course of water turbidity and its rate of removal in the overflow trough water by H. schlegeli clams. The rate of turbidity (as a percentage) was calculated as described in the text. Means and standard deviations are shown based on results from five replicates. ▨, turbidity with clams; □, turbidity without clams (control); ●, rate of turbidity removal by clams.

Time course of the density of microbes and their rates of removal from the overflow trough water by H. schlegeli clams. The rate of microbe removal (as a percentage) was calculated as described in the text. Means and standard deviations are shown based on results from five replicates. The absence of bars indicates that the error was smaller than the symbol. (A) Heterochromulina sp. (class Phytomastigophora); (B) Gymnamoebia sp. (class Lobesea); (C) Testacealobosia sp. (class Lobesea); (D) Vorticella sp. (class Ciliatea). ▨, density of microbes with clams present; □, density of microbes without clam present (control); ●, rate of microbe removal by the clams.